We present a simple phenomenological model to describe the phase equilibria and structural properties of microemulsions. Space is divided into cells of side xgr;; each cell is filled with either pure water or oil. Surfactant molecules are presumed to form an incompressible fluid monolayer at the oilndash;water interface. The monolayer is characterized by a sizehyphen;dependent bending constantK(xgr;), which is small for xgr;ge;xgr;K, the de Gennesndash;Taupin persistence length. The model predicts a middlehyphen;phase microemulsion of structural length scale xgr;ape;xgr;Kwhich coexists with dilute phases of surfactant in oil and surfactant in water. (These phases have xgr;ape;a,abeing a molecular length.) On the same ternary phase diagram, we find also two regions of twohyphen;phase equilibrium involving upperhyphen; and lowerhyphen;phase microemulsions that coexist with either almost pure water or oil. At low temperatures and/or high values of the bare bending constant,K0equiv;K(a), the middlehyphen;phase microemulsion may be entirely precluded by separation to a lamellar phase, whereas at high temperature and/or low values ofK0, there is a firsthyphen;order transition between a disordered microemulsion and a lamellar phase. In the absence of spontaneous curvature the phase diagram is oilndash;water symmetric. It may be asymmetrized by: (i) spontaneous curvature in the middle phase or (ii) a difference between the free energy of the two dilute phases. If the asymmetry is sufficiently large, the threehyphen;phase region disappears.
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